Found LinuxPackages.net which is a repository of all kinds of packages built for Slackware. Useful for when you don’t have time to build something from source.
Next, I still have a couple of servers running Red Hat 7.3, which was retired by Red Hat last year. This meant that they couldn’t be updated by regular means. The Fedora Legacy project provides these crucial updates so that the life of these systems can be extended. You’ll need to install Yum for easier updating but you could just download the packages and manually update. I just gave it a try and it downloaded quite a few header files but didn’t seem to update anything. Will have to check the configuration.